A liquid crystal display (LCD) is a flat-panel display that has a number of advantageous features including high resolution, drastically reduced thickness and weight, and low power dissipation. The LCD market has been rapidly expanding recently as a result of tremendous improvements in its display performance, significant increases in its productivity, and a noticeable rise in its cost effectiveness over competing technologies.
A twisted-nematic (TN) mode liquid crystal display device, which used to be used extensively in the past, is subjected to an alignment treatment such that the major axes of its liquid crystal molecules, exhibiting positive dielectric anisotropy, are substantially parallel to the respective principal surfaces of upper and lower substrates and are twisted by about 90 degrees in the thickness direction of the liquid crystal layer between the upper and lower substrates. When a voltage is applied to the liquid crystal layer, the liquid crystal molecules change their orientation directions into a direction that is parallel to the electric field applied. As a result, the twisted orientation disappears. The TN mode liquid crystal display device utilizes variation in the optical rotatory characteristic of its liquid crystal layer due to the change of orientation directions of the liquid crystal molecules in response to the voltage applied, thereby controlling the quantity of light transmitted.
The TN mode liquid crystal display device allows a broad enough manufacturing margin and achieves high productivity. However, the display performance (e.g., the viewing angle characteristic, in particular) thereof is not fully satisfactory. More specifically, when an image on the screen of the TN mode liquid crystal display device is viewed obliquely, the contrast ratio of the image decreases significantly. In that case, even an image, of which the grayscales ranging from black to white are clearly observable when the image is viewed straightforward, loses much of the difference in luminance between those grayscales when viewed obliquely. Furthermore, the grayscale characteristic of the image being displayed thereon may sometimes invert itself. That is to say, a portion of an image, which looks darker when viewed straight, may look brighter when viewed obliquely. This is a so-called “grayscale inversion phenomenon”.
To improve the viewing angle characteristic of such a TN mode liquid crystal display device, an inplane switching (IPS) mode liquid crystal display device, a multi-domain vertical aligned (MVA) mode liquid crystal display device, an axisymmetric aligned (ASM) mode liquid crystal display device, and other types of liquid crystal display devices were developed recently. Liquid crystal displays employing any of the novel modes described above (wide viewing angle modes) solve the concrete problems with viewing angle characteristics, specifically, the problems that the display contrast ratio decreases considerably or the grayscales invert when the display surface of the display is viewed obliquely.
Although the display qualities of LCDs have been further improved nowadays, a viewing angle characteristic problem in a different phase has arisen just recently. Specifically, the γ characteristic of LCDs would vary with the viewing angle. That is to say, the γ characteristic when an image on the screen is viewed straight is different from the characteristic when it is viewed obliquely. As used herein, the “γ characteristic” refers to the grayscale dependence of display luminance. That is why if the γ characteristic when the image is viewed straight is different from the characteristic when the same image is viewed obliquely, then it means that the grayscale display state changes according to the viewing direction. This is a serious problem particularly when a still picture such as a photo is presented or when a TV program is displayed.
According to a known method, such viewing angle dependence of the γ characteristic can be reduced by providing two or more subpixels for each single pixel and by making the luminance of one of the two subpixels different from that of the other when a moderate luminance is displayed (see Patent Documents Nos. 1 and 2, for example).
Specifically, the liquid crystal display device disclosed in Patent Document No. 1 applies a different effective voltage to the liquid crystal layer of a second subpixel from the one applied to the liquid crystal layer of a first subpixel when a moderate luminance is displayed, thereby making the luminances of the first and second subpixels different from each other and reducing the viewing angle dependence of the γ characteristic. The transmittance of the liquid crystal layer changes with the absolute value of the effective voltage irrespective of the direction of the electric field applied to the liquid crystal layer (i.e., the direction of the electric line of force). Thus, the liquid crystal display device disclosed in Patent Document No. 1 inverts the direction of the electric field applied to the liquid crystal layer alternately every vertical scanning period, thereby flattening the uneven distribution of DC levels and overcoming residual image and other reliability-related problems.
Meanwhile, the liquid crystal display device disclosed in Patent Document No. 2 inverts the brightness levels of first and second subpixels every vertical scanning period (e.g., makes the luminance of the first subpixel higher than that of the second subpixel in a first vertical scanning period but makes the luminance of the second subpixel higher than that of the first subpixel in a second vertical scanning period). In addition, the device also inverts the direction of the electric field applied to the liquid crystal layer every vertical scanning period, too. If one of multiple subpixels were always bright, then the image on the screen would look non-smooth. However, the liquid crystal display device disclosed in Patent Document No. 2 minimizes such non-smoothness of the image on the screen by inverting the brightness levels of the first and second subpixels one vertical scanning period after another.
It should be noted that such a display or driving method that reduces the viewing angle dependence of the y characteristic by making the luminances of multiple subpixels different from each other will be referred to herein as a “multi-pixel display”, a “multi-pixel drive”, an “area grayscale display” or an “area grayscale drive”.                Patent Document No. 1: Japanese Patent Application Laid-Open Publication No. 2004-62146 (corresponding to U.S. Pat. No. 6,958,791)        Patent Document No. 2: Japanese Patent Application Laid-Open Publication No. 2003-295160        